The present invention relates to a method for making beam leads for interconnecting electronic packages to a substrate and more particularly to a method for making thick aluminum (0.001 to 0.002 inch) beam leads.
One heretofore known method of making thick aluminum beam leads consisted of vacuum depositing up to 0.001 inch of aluminum on both sides of an organic plastic film. A window was etched in the aluminum on one side of the film and, simultaneously, a lead pattern was etched in the aluminum on the opposite side of the film. The exposed film was then etched away leaving leads bridging a window. This method has several advantages over other process in that only one aluminum etching step was required and also the aluminum leads were etched while they were still supported by the plastic film.
The use of plastic film in a vacuum-depositing process, however, has various shortcomings. Successful vacuum deposition of aluminum on organic plastic film was very sensitive to cleaning procedures. Also 1 mil thick aluminum was difficult to etch and loss of adhesion was encountered frequently. Because of etching difficulties, thicknesses greater than 1 mil were impossible to achieve. Additionally, the etching process was very sensitive to the deposition rate of the aluminum. Aluminum deposited by a slow rate etched better than aluminum deposited by a faster rate, however, slower deposition rates produced more brittle aluminum than faster rates. Consequently, when the leads were bonded to components, the pull strength of the aluminum leads made by a slow deposition rate was lowered.